In computer driven color printing and plotting, particularly in situations involving large images, it is convenient for space-saving and web-control reasons to move a paper or film web back and forth from a supply roll to a takeup roll. Between paper supply and takeup rolls an electrostatic writing head forms latent images, one color at a time, with all colors in registration, and a downstream toner station develops the latent images, which overlay each other to form a single color image.
The writing head may be a small writing head of the scanning or shuttle type, moving back and forth across the width of the web, or it may be a full-width head. The toning station provides yellow, magenta, cyan and black toner in separate toning passes, one color for each pass of the web in the forward direction. The web is moved back and forth between the takeup roll and the supply roll, but toner is supplied to the latent image only in the forward direction of travel, the reverse travel sometimes being used for web registration measurements.
In order to economize on the cost of the electronics, the plotting or printing speed is allowed to vary. In areas where the latent image being formed is relatively complex, the paper speed is slowed in order to allow data processing or rasterizing by a relatively inexpensive processor to keep up with the demand. A similar process occurs in facsimile machines where transmission automatically slows for complex patterns but is considerably faster for blank or low information contents paper to maximize throughput. Typically, the web speed can vary by a factor of ten or so.
Paper speed and position can be controlled through the use of a precision shaft encoder attached either to the takeup roll or to an idler wheel running on the paper surface together with tracking marks written along one or both edges of the web, outside the image areas. Generally, the more stringent are the accuracy and color dot registration requirements, the more complex is the encoder/marking system used for web position control. For good image quality, it is necessary to use regular or periodic marks, usually black, along either one or both web edges.
The marks are read optically and the resulting position information is used to control both the motors driving the paper motion as well as the timing of the deposition of latent image charge by the writing head. The marks may also be used to correct lateral, i.e. across the web, position of the writing so that the image being created follows lateral web positional errors. It is exceedingly difficult to make paper track accurately in multi-pass printers and plotters. The use of tracking marks has been found very useful in correcting these tracking errors by physically moving the write head laterally in such a manner that it "follows" the web position.
In the case of a scanning or shuttle type writing head, only the timing of the charge deposition need be varied to correct lateral web position errors.
Because the web is immediately toned after latent image generation, the speed of the web varies as it passes over the toner applicator just as it does in the area of the write head. Toners have been formulated so as to be tolerant of this speed variation, i.e., the colors come out more or less the same regardless of speed variations through the toning station.
Considerable sacrifice in color saturation and some variation in hue are the price paid for this variable speed toning. If the toning were done at a single, predetermined web speed, a toner set could be formulated which would give better colors and better color uniformity. With the increasing non-engineering applications of computer driven printing and plotting technology, which require picture quality comparable to photography, the advantages of constant-speed toning are becoming ever more important.
The problems with variable speed toning arise from the need to completely neutralize the image electrically during toning. If any latent image charge remains after toning with a given color, it will still be present when a subsequent color is used for toning and it will cause deposition of the subsequent color on top of the previous toned layer where it is not wanted. As an example, an image portion may be cyan. If the cyan latent image is incompletely neutralized during cyan toning, some of the "cyan" latent charge will remain and, during a subsequent magenta toning pass, magenta toner particles will deposit on the "cyan" area causing color contamination and serious darkening of the desired cyan color. In fact, every color has the potential problem of being so contaminated by the subsequent color pass if there is any such "residual potential". This problem is so serious that it has led to a special color sequence, namely BCMY (black, cyan, magenta, and yellow) for most electrostatic applications. By going, generally, from darker to lighter colors in sequence, the contamination effect is that of a lighter color on top of a darker one and this is less serious than the other way around. In other color imaging technologies, this is unnecessary. The final color employed in electrostatic printing, usually yellow, can exhibit a high residual potential without harm as there is no subsequent toning pass.
Residual potential problems and color contamination can be minimized only by increasing the electrical conductivity of the toner. Only in this way can it be assured that the residual potential is held within acceptable limits. Latent image charge neutralization is a function of the toner's electrical conductivity and the effectiveness of the particular toner applicator design. If either the toner is too insulating or the applicator is not sufficiently effective, excess residual potential will cause serious color degradation. Generally, the amount of residual charge which can be tolerated is about 5 percent of the original image charge level, i.e. at least 95 percent of the latent image must be neutralized in the act of toning with a given color other than the final color. The requirement varies with application, however, and the requirement may reach 98 percent neutralization in some cases or even more.
When printing at slow speeds, as in the case of very complex images, this neutralization requirement is easily met, but when the printed area is simple, allowing high web speeds, it is difficult to adequately neutralize the latent image by toning. The problem is worst for printed images which include both simple/fast areas and complex/slow areas. This is because the human eye is especially sensitive to color variations which are not supposed to be there. This problem has led to serious compromises in toner formulation as explained below.
In an electrostatic toner there are always charged and colored particles which are the desired materials for creating the visible colored output image. There are also charged but colorless ions which can help neutralize the latent image and reduce the residual potential, but they contribute nothing to the color on the output print. These ions actually detract from the desired color because they substitute for the toner particles, using up some to the latent image without producing any visible results. While some level of ions are necessarily present in any liquid toner, variable speed toning has led directly to a high level of ions as compared to colored particles for two different reasons. First, a high level of ions helps reduce residual potential by assisting the charged, colored particles in neutralizing the latent image charge. In addition, it has been found that the ions can, if the toning process is very prolonged, remove some of the already deposited colored particles by substituting for them. This sounds undesirable, and it is, but it is useful in keeping the color more or less uniform as toning is prolonged at very slow web speeds. Rather than getting more and more color saturation in very complex areas which run very slowly, the removal process compensates for the particles which are slowly depositing. The net effect is that colors can be more-or-less unchanged even when the web speed varies by a factor of five or more. It is not uncommon to have speed variations of ten to one, in fact.
It might be thought that complete charge neutralization could be achieved simply by increasing the number of colored particles, i.e. the concentration of the toner without adding ions which detract from the image. While it is true that simply adding more charged particles does increase toner conductivity and reduce residual potential, unfortunately, excess colored particles also contribute to background staining and this limits the "solids content" of practical toner formulations. The real solution is to formulate toners with fewer ions so the particles have less competition for the available image charge. This requires toning at relatively fixed speed. By restricting toning speed variations to a narrow range of some ten or twenty percent, the level of ions can be dramatically reduced leading to image benefits.
Colored toners which behave in a speed-independent manner are supplied by Hilord Chemical Corporation, Hauppauge, N.Y. and by Specialty Toner Corporation, Fairfield, N.J. Such colored sets of toners are formulated for specific printers and plotters because of the variation in effectiveness of different applicator designs, i.e. less effective applicators require more ions in order to achieve this speed independence. If the applicator is made more effective, generally at higher cost, the problem can be minimized but the basic source of the difficulty is the varying speed of the toning process caused by the economies of image creation at variable speed. Only by toning at a fixed and predetermined web speed can the level of ions in the toner be minimized and maximum color performance achieved. A toner set which is specifically formulated for a given applicator which is operated at a fixed web speed is truly the best that can be done with that applicator. Color saturation is optimized and there is no chance for speed variations or excess residual potentials to occur. It is therefore highly desirable to write the latent image at variable speed so as reduce equipment cost and it is equally desirable to tone that latent image at a fixed and predetermined web speed.
In principle a slack-loop of web could be accumulated between the write head and the toning station so as to permit constant-speed toning while keeping the cost advantages of variable speed image creation. In practice, this is impractical because the future complexity of the plot cannot be predicted and, essentially, the whole latent image would have to be accumulated and the toning started after creating the entire latent image. The mechanical problems of accumulating up to 40 feet of paper or film in a slack-loop are also daunting.
An object of the invention was to devise a more efficient toning process with better color saturation and hue properties.